The present invention is first of all an advance in the art of film-covered, exposed-lens retroreflective sheeting such as taught in McKenzie, U.S. Pat. No. 3,190,178. Such sheeting, which offers the brightest retroreflection of any known retroreflective sheeting made from glass microspheres, comprises (1) a base sheet in which a dense monolayer of transparent microspheres is partially embedded and partially exposed, with a specular reflective metal layer underlying the embedded surfaces of the microspheres, (2) a transparent cover film disposed in spaced relation above the layer of microspheres, and (3) a network of narrow, intersecting polymer-based bonds that extend over the surface of the base sheet to adhere the base sheet and cover film together and to divide the space between the base sheet and cover film into hermetically sealed cells or pockets in which the microspheres have an air interface. This "exposed-lens" construction (that is, with the microspheres having an air interface) is responsible for the bright retroreflection provided by such sheeting.
A special challenge with such sheeting is to obtain lasting bonds between the cover film and base sheet. The bonds in existing commercial sheeting have been susceptible to largely two kinds of disruption: (1) disruption caused by the heat and pressure used to apply reflective sheeting to a base substrate such as a traffic sign blank, and (2) disruption caused by outdoor weathering, including extremes of temperature cycling; rain, snow, ice and other forms of precipitation or moisture; and sunlight. Upon failure of the bonds, moisture can cover the exposed faces of the microspheres, whereupon the microspheres do not focus light rays onto the specular reflective layer on the back of the microspheres as they otherwise would, and retroreflection is very greatly reduced. The utility of film-covered exposed-lens retroreflective sheeting would be greatly expanded if some way were found to provide bonds of greater durability.
Another hermetically sealed cellular reflective sheeting that could benefit from improved bond strength between a cover film and base sheet is so-called "cube-corner" sheeting. Some varieties of cube-corner sheeting include a clear, transparent base sheet having a flat front surface, which serves as the front face of the sheeting, and a rear surface configured with cube-corner elements. A cover film is desired at the rear of the sheeting to maintain an air-interface for the cube-corner elements and also to provide a flat rear surface for bonding the sheeting to a substrate. A network of bonds as described above is potentially useful to hold the cover film to the base sheet, but again these bonds should provide a more lasting hermetic seal than has been available in the past.